In foliar feeding of plant nutrients, the nutrients are dissolved in water and the aqueous solution of nutrients is sprayed on the leaves of the plants. U.S. Pat. No. 3,087,806 describes a method for improving the yield of soybeans by spraying the plants periodically throughout their growth with an aqueous solution of urea phosphate. The patent recommends twice weekly applications, and in an example such twice weekly sprayings were carried out for a period of 18 weeks.
U.S. Pat. No. 3,558,300 describes a method for foliar feeding of field crops including soybeans and cereal grains, with an aqueous solution of an ammonium polyphosphate. Adequate ground fertilizer is employed in conjunction with the foliar feeding. It is claimed that the method improves the stress resistance of the plants. The ammonium polyphosphate solution is described as being applied during the crop growth period up to the flowering period.
Unsuccessful results from foliar fertilization in increasing seed yield have been reported by Mederski and Volk for wheat, corn, soybeans, oats and alfalfa. When the field crops were grown in soils with adequate fertility they failed to respond positively to foliar sprays containing N, P, and K. Mederski, H. J. and Volk, G. W. Foliar Fertilization of Field Crops, Ohio Agr. Exp. Sta. Research Cir. 35, (Aug. 1956). It has been generally assumed that legume and cereal grain crops which have adequate ground fertilization will not produce significantly higher yields of the grain by supplemental foliar fertilization, although it has been recognized that the nutrients in the spray are absorbed by the leaves of the plants.
During the vegetative period of plant growth, the amount of nutrients that can be applied as a foliar spray without causing severe "burning" of the leave is not sufficient in itself to supply the nutrient needs of the grain crop plants for very long periods. Consequently, it has been thought necessary to make frequent applications of foliar sprays over the entire growing season to maintain the nutrient content of the plants. Moreover, where the soil contains adequate available nutrients, plants during their vegetative period of growth feed effectively on the soil nutrients. Therefore, for legume and cereal crops fertilizers have been applied to the soil to supply all the required nutrients. Nutrient deficiencies in the soil can be readily corrected by varying the formulas of the fertilizer. Consequently, no need has been generally recognized for supplemental foliar feeding. It has been stated in general that "foliar feeding is often effective when roots are unable to absorb sufficient nutrients from the soil," and that "at flowering many crop plants, having achieved their maximum leaf surface, show a marked depression in general overall metabollic activity, including nutrient uptake by the roots," suggesting that "foliar applications of nutrients should be especially beneficial under such conditions." Wittwar, S. G., Bukovac, M. J., Tukey, H. B., "Advances in Foliar Feeding of Plant Nutrients," Chap. 13, p. 447, Fertilizer Technology and Usage (1963, Soil Sc. Soc. of Amer, Madison, Wisc.).
However, prior to the present invention only limited success has been achieved with the foliar feeding of legume and cereal grain crops. For soybeans, the focus of research has been on the weeks immediately prior to the onset of flowering. This period is believed by some to be a vegetative growth stage in which the soybean crop needs more N than the soil can usually provide. deMooy, C. J., Pesek, J., Spaldon, E., "Mineral Nutrition," Chap. 9, p. 280, Soybeans: Improvement, Production, and Uses, No. 16 in The Series Agronomy (1973, Amer. Soc. Agron., Madison, Wisc.). Iwata and Utada applied N in various stages of growth, and found at soybean yields were reduced most by withholding N during the period 2 to 3 weeks prior to flowering. Withholding N for one month before this critical period, or for any 2-week period after flowering, did not lead to any large reduction in yield. Iwata, M. and Utada, A. J. Jap. Soc. Hort. Sci. (1967, Tokyo) 37 (1): 57-66 (J.e). Neunylov and Slabko also have reported that N supplied just prior to flowering gave positive responses, including more abundant flowering and higher yield of soybeans. N application after the onset of flowering did not give the same effect. Neunylov, B.A., and Y.I. Slabko, Agrokhimiya 11:45-51 (1967).
In some recent tests at Iowa State University, Ames, Iowa, Barel and Black found that corn and soybeans were more tolerant of the foliar application of polyphosphates than orthophosphate. Barel, D. and Black, C. A., "A New Wrinkle in Foliar Feeding of Phosphorus," Iowa State University Ext. Ser. Bull. EC-981 (Jan., 1975). As part of a test procedure, Barel and Black grew soybeans plants to maturity in soil treated with N, K, and P, and the plants were sprayed twice with an aqueous solution of the phosphorus compounds, first when the plants were five weeks old and again when the filling of pods were underway. The yields from the foliar treated plants were found to significantly exceed the yields of the unsprayed controls for most of the P compounds tested.